LONDON, Oct 17 (Reuters) - Merlin Entertainments, operator of tourist attractions such as Madame Tussauds waxworks, warned on full-year earnings on Tuesday, blaming a dip in trading in its key summer period on a series of attacks in Britain, sending its shares tumbling.

The stock fell up to 21 percent after Merlin forecast core earnings for 2017 in the range of 470 million pounds to 480 million pounds ($622-$636 million).

Analysts on average had forecast 490 million pounds, according to Reuters data. The firm made core earnings of 433 million pounds in 2016.

Merlin, which also runs the London Eye, Legoland and theme parks such as Alton Towers in Britain, said that trading in recent weeks had remained mixed and group like-for-like revenue growth for 2017 was now expected to be “approximately flat” on 2016.

“The spate of terror attacks witnessed in the UK marked an inflection point in Midway London and UK theme park trading,” said Chief Executive Nick Varney.

“Poor weather in Northern Europe and extreme weather in Italy and Florida also impacted peak season trading.”

Britain has seen five attacks since March, described by police as terrorism. The current official security threat level in Britain is “severe”, meaning that more attacks are considered highly likely.

For the 40 weeks to Oct. 7 Merlin said group like-for-like revenue was up just 0.3 percent, reflecting the difficult trading at its London attractions and European theme parks, where like-for-like revenue fell 1.0 percent and 2.1 percent, respectively.

Merlin said it would reallocate capital investment to address the ongoing volatile market environment and underlying cost pressures.

It plans to reduce spending on its existing estate by 100 million pounds over the 2018-21 period.

Merlin’s total revenue growth was 12.4 percent at actual exchange rates and 5.9 percent at constant currency rates, driven by new business development, including the opening of LEGOLAND Japan, five new Midway attractions, and 381 new accommodation rooms.

“Whilst it is too early to predict the outlook for 2018, it is likely that the recent trends experienced in London will persist for the foreseeable future,” Merlin said. ($1 = 0.7553 pounds) (Reporting by James Davey; Editing by Paul Sandle and Louise Heavens)